This invention relates semiconductor substrate processing by the application of titanium coatings, particularly by plasma enhanced chemical vapor deposition (PECVD) methods. The invention more particularly relates to the cleaning and stabilizing of CVD reactors used in such processing and to the passivating and conditioning of such reactors following such cleaning and to maintaining the reactors in a stable state during the subsequent use of the reactors for Ti-PECVD processing of semiconductor substrates.
Chemical vapor deposition (CVD), and particularly plasma enhanced chemical vapor deposition (PECVD) are processes that are being increasingly used in the application of titanium (Ti) or titanium-containing films to substrates in semiconductor manufacture. One such Ti-PECVD process is at least theoretically available to deposit titanium on contacts of features, particularly high aspect ratio features, on semiconductor wafers. In investigating contact level metallurgy in the course of arriving at the present invention, applicants have determined that production applications with such processes involve issues of process uniformity, process repeatability and process stability that are as important as the fundamental film properties and deposition characteristics.
One such issue is the cleaning methodology for the chamber of the reactor in which the CVD is carried out. Such reactors must be treated to remove accumulated reactants, reaction products and reaction by-products from the reactor surfaces. Materials that collect on these surfaces during the use of the reactor for Ti-CVD are often sources of peeling and contamination in the chamber, leading to a high number of particles which cause contamination of the surfaces of the wafers being processed resulting in interference with critical process reactions on the wafer. Also, accumulation of this material on the surfaces of a reactor can cause long term drifts in process parameters leading to unstable or unpredictable process performance and degraded process results. Furthermore, many CVD reactors that are available for Ti-CVD are provided with nickel alloy susceptors on which the wafers are supported for processing. Silicon wafers have a greater tendency to stick to a nickel alloy susceptor following the cleaning of the susceptor.
When the conditions of surfaces in a CVD reactor used for Ti-PECVD are changed, such as by cleaning the reactor to remove deposits that accumulate during deposition, changes occur in the deposition processes which stabilize only after some form of chamber conditioning. Applicants"" empirical observation is that, following such changes in reactor conditions, there is either some amount of reactor operating time that must transpire or some number of wafers that must be processed in a reactor before the process stabilizes. Applicants believe that this effect is due to a change in the state of the reactor surfaces, due in part to film deposition on the surfaces which alters thermal emissivity, adhesive properties, electrical conductivity or other properties that directly affect the results on the processed wafers. It is desirable to minimize the amount of time devoted to the initial conditioning of a reactor following cleaning or other such condition changes of the reactor before the introduction of wafers into the chamber for processing, particularly in a manufacturing environment.
Deposition chambers are typically cleaned in one of two ways: 1) In situ cleaning, by which reactor surfaces are cleaned without opening the system to air, and preferably without cooling any parts of the chamber, and 2) wet cleaning, which generally entails cooling the reactor components, opening the system and wiping or scrubbing the reactor components with water or other chemicals to remove deposits from them. For both of these methods, applicants observe that the process must be recovered, that is, the chamber should be reconditioned, to stable baseline performance after the cleaning procedure has been carried out.
The Ti-PECVD process uses TiCl4 and H2 in a reduction reaction conducted in a plasma environment to form metallic Ti as the main reaction product and HCl as the main reaction by-product. During the course of this reduction reaction, other by-products may form, such as TiClx, with x less than 4. These products, along with metallic Ti, may be deposited on reactor surfaces to an extent that is strongly related to the chamber geometry and to the temperature distribution on the reaction chamber surfaces. Hot metallic surfaces, such as hot nickel alloy surfaces for example, when directly exposed to TiCl4, have a tendency for metal chlorides to form thereon that may have a detrimental effect on process performance. For the PHOENIX(trademark) system of applicants"" assignee, for example, such reactor surfaces on which such undesirable deposition may tend to occur are the face of the substrate-supporting susceptor, the face of the process gas dispersing showerhead, and a limited region on the reactor walls located near the plane of the wafer and below. The composition of deposited material is related to the temperature of the surface on which it deposits and to the concentration of various reaction species, reactants, reaction products and reaction by-products at such surface.
For example, during continuous operation of a Ti-PECVD process, Ti bearing films accumulate on the internal surfaces of the reactor. The composition of these films range from Ti rich on the hotter surfaces to Cl rich on the colder surfaces of the reactor. These films are intrinsically unstable. The Ti rich films oxidize over time upon exposure to the residual water and oxygen that are present in the chamber. Since this oxidation has, in the prior art, been a highly uncontrolled process, it has been regarded as undesirable. During oxidation of these Ti rich films, their physical properties change from electrically conducting to electrically insulating, resulting in unstable and otherwise changing plasma and other characteristics within the chamber during the performance of coating processes. Cl rich films, on the other hand, have a relatively high vapor pressure and result in an uncontrolled background of TiClx(x less than 4) in the chamber. These TiClx species contribute to the deposition reaction and result in unstable process characteristics.
The material that deposits on the walls and other reactor surfaces during the formation of titanium is very hygroscopic and deliquescent, reacting with residual water vapor and O2, when present, to form a TiO2-based film. The gettering properties of such Ti films are well known. TiO2 film has the properties of being chemically stable and electrically non-conductive. Where a reactor having a titanium rich coating on its internal components must be opened for cleaning, these reactions accelerate, producing airborne reaction by-products and heat, which are hazards that must be controlled.
Titanium films that are deposited onto semiconductor wafers are usually followed by a passivation process by which a passivating layer of a stable substance such as titanium nitride is deposited on the surface of the titanium film. Where the Ti deposition process is one of CVD, the TiN film is formed in a dedicated reactor by the reaction of titanium with ammonia. The formations of the Ti and overlying TiN films is carried out in a multiple reactor tool having a Ti-CVD reactor and a TiN-CVD reactor connected to a transfer module through which wafers are transferred from the Ti-CVD module to the TiN-CVD module for successive processing.
There exists a need to more efficiently and effectively condition a reactor, particularly one used for the PECVD of titanium, following the cleaning of the reactor.
A primary objective of the present invention is to provide stable process performance in a Ti-PECVD apparatus.
A particular objective of the present invention is to provide a Ti-PECVD method and apparatus which maintains the reactor in a stable condition and maintains the stable performance of the Ti-PECVD process during continuous operation, particularly under semiconductor manufacturing operations.
Particular objectives of the present invention are to provide for the breaking in and stabilization of a PECVD reactor for Ti-PECVD processing following changes in reactor conditions, particularly following the changes in conditions that are brought about by the cleaning of internal reactor surfaces, such as by wet cleaning or by in-situ cleaning, and where the reactor is either opened or remains closed during a cleaning operation.
A further objective of the present invention is to provide an in-situ PECVD reactor cleaning process that facilitates reactor break-in, reactor and process recovery and process stabilization.
Certain aspects of the present invention are based in part upon the determination that chamber stabilization by oxidation or reduction/passivation of internal reactor surfaces, eliminates an uncontrolled source of Ti-containing species that are volatile enough to reach the wafer surface.
A particular objective of the present invention is to more efficiently and effectively condition a reactor following the cleaning thereof, particularly where such reactor is used for the PECVD of titanium.
A further objective of the present invention is to improve the efficiency of, and reduce the equipment required for, performance of successive Ti-CVD and TiN-CVD processes.
According to principles of the present invention, stabilized titanium bearing film is provided in a Ti-PECVD reactor as changes in reactor conditions occur. Such changes in reactor conditions include those brought about by the cleaning of internal reactor components which, for example, remove titanium bearing deposits from the surfaces of reactor components as well as those brought about by the use of the reactor, for example, to perform a Ti-PECVD process on a wafer which results in the formation of Ti bearing film deposits on reactor components.
In accordance with certain embodiments of the invention, a controlled oxidization or reduction/passivation of Ti bearing films is carried out in a Ti-PECVD reactor following changes in the nature or extent of Ti bearing films on the reactor components. In certain embodiments of the invention, a controlled deposition of reactants, reaction products, reaction by-products or related materials is provided in the chamber following the cleaning of the chamber. In other embodiments of the invention, a controlled presence of one or more oxidizing or reducing agents is provided in the chamber in the early stages of recovery following a cleaning of the chamber. Still in further embodiments of the invention, films formed on the surfaces of reactor components are oxidized or reduced, or are passivated, in a controlled way following the performance of a Ti-PECVD process in the reactor to maintain, in a stabilized state, the film that may deposit on the components during the performance of such process. Preferably, this controlled oxidation or reduction and passivation is carried out after every Ti-PECVD deposition onto a single substrate or at least after a selected number of Ti-PECVD depositions onto a series of substrates.
In one preferred embodiment of the invention, a reactor is opened to the atmosphere and wet cleaned to remove Ti bearing films that have accumulated on the surfaces of reactor components. The reactor is then closed and a vacuum is restored within the reactor. The components of the reactor are then exposed, for a time, to a plasma formed in a mixture of argon and hydrogen gas within the chamber. Particularly, the gas introduction system, preferably in the form of a showerhead, is exposed, for a time of, for example, 1 to 5 minutes, to the H2/Ar plasma. The exposure of the showerhead and other components to the plasma removes contaminants from the component surfaces.
Preferably, the reactor is run through the steps of a Ti-PECVD process but without a wafer in the chamber to pre-coat reactor components with a Ti bearing film. Where the susceptor is made of a metal such as a nickel alloy, the formation of such a pre-coating on the surface of the susceptor prevents the sticking of silicon wafers to the hot susceptor. Preferably, the showerhead is preheated to a temperature that, for a Ti-PECVD reactor or process, is preferably higher than approximately 425xc2x0 C. An effect of the heating of the showerhead is to improve the adhesion of the Ti bearing film that is deposited when this initial Ti-PECVD process is run in the reactor immediately following the H2/Ar plasma cleaning.
Following the pre-coating of the reactor components, a fast process break-in of the reactor is accomplished by forming a H2/Ar plasma in the reactor before TiCl4 is introduced into the reactor. One advantage of first forming the H2/Ar plasma is that direct exposure of hot metallic surfaces to the TiCl4 gas is prevented.
In accordance with certain embodiments of the invention, a controlled oxidizing or reducing of Ti bearing films, such as by nitridizing or otherwise passivating the film, is carried out in a Ti-PECVD reactor following changes in the nature or extent of Ti bearing films on the reactor components. In certain embodiments of the invention, a controlled deposition of reactants, reaction products, reaction by-products or related materials is provided in the chamber following the cleaning of the chamber. In other embodiments of the invention, a controlled presence of one or more oxidizing or reducing agents is provided in the chamber in the early stages of recovery following a cleaning of the chamber. Still in further embodiments of the invention, films formed on the surfaces of reactor components are oxidized or reduced in a controlled way following the performance of a Ti-PECVD process in the reactor to stabilize the film that may deposit on the components during the performance of such process. Preferably, controlled oxidation or reduction and passivation is carried out after every Ti-PECVD deposition onto a single substrate or after a selected number of Ti-PECVD depositions onto a series of substrates.
In another preferred embodiment of the invention, the reactor is cleaned in situ, without being opened to the atmosphere. Such a cleaning is usually conducted by the introduction of fluorine or chlorine-containing gases into the chamber, frequently forming a plasma therewith. Examples of such gases are nitrogen trifluoride (NF3), trifluoro-chlorine (ClF3) or chlorine (Cl2). The in situ cleaning process is carried out to remove Ti bearing films and other contaminants that have accumulated on the surfaces of reactor components. Following such in situ cleaning, all such cleaning gases and reaction products are removed from the chamber. The cleaning includes exposing the components of the reactor, for a time, to a plasma formed in a mixture of argon, hydrogen and ammonia gases, for example, as explained following the wet cleaning process discussed above. This plasma cleaning is carried out for 1 to 10 minutes or as is necessary to remove fluorine and chlorine-containing species from the chamber. Then the reactor chamber is pump purged at least five times.
The reactor is, preferably, then run through the steps of a Ti-PECVD process without a wafer in the chamber to pre-coat reactor components with a Ti bearing film, as is done following the wet cleaning process described above. Also, following the pre-coating of the reactor components, a fast process break-in of the reactor is accomplished by forming a H2/Ar plasma in the reactor before TiCl4 is introduced into the reactor. One advantage of first forming the H2/Ar plasma is that direct exposure of hot metallic surfaces to the TiCl4 gas is prevented.
In a further preferred embodiment of the invention, the Ti-PECVD process and reactor are maintained stable during the continuous operation of the reactor and performance of the process by performing stabilization steps following individual depositions onto wafers. Ti bearing films that accumulate on reactor components during continuous operation of a Ti-PECVD process that can vary in composition and can result in constant change of reactor and process conditions are stabilized by oxidizing or reducing the films and passivating the films, as appropriate, in a controlled way following individual depositions onto wafers in the chamber, either following a selected number of depositions or, preferably, following each performance of a Ti-PECVD process on a wafer and before each entry of a new wafer into the chamber for similar Ti-PECVD processing.
The stabilization steps include the introduction of controlled quantities of gases such as oxygen (O2) or water vapor (H2O), for example, to form an oxidation reaction with the newly deposited films on the reactor surfaces or the introduction of controlled quantities of gases such as hydrogen (H2), ammonia (NH3), silane (SiH4), methane (CH4) or di-borane (B2H6), for example, to form a reduction reaction. The gas is caused to flow in the chamber. A plasma may also be formed with the gas to enhance the reaction. NH3 in particular is preferred for the stabilization process and introduced into the chamber with argon energized to an RF plasma, with the susceptor and showerhead heated. The stabilization step following each Ti-PECVD processing of a wafer may typically range from 10 to 60 seconds, depending on conditions.
In a preferred embodiment of the invention, a passivation step is performed in the reactor following each Ti deposition. The passivation step is performed before the wafer on which the Ti film was deposited has been removed from the chamber, such that a subsequent Ti passivation step is performed on Ti film deposited on the wafer at the same time that the passivation step is performed on the Ti film deposited on the reactor components. Where the passivation of the wafer by the formation of a TiN film is desired, following the Ti deposition reaction, NH3 is introduced into the Ti-PECVD reactor chamber and reacted to form a TiN film over the Ti films on the wafer and the reactor components, preferably in a plasma enhanced process. Preferably also, a hydrogen-argon plasma is formed in the reactor with the NH3 being introduced when the temperature of the reactor showerhead is at least at 425xc2x0 C., with the susceptor temperature at about 630xc2x0 C., the wafer at about 590xc2x0 C. and the walls of the reactor at between 100xc2x0 C. and 200xc2x0 C.
Alternative passivation gases may be used for the chamber passivation process. Where such gases are not compatible with the processing of the wafer, the wafer may be removed from the chamber and the passivation carried out in the chamber without a wafer present. In such cases, where the film accumulated in the chamber is light, passivation of the chamber may be carried out at intervals following the Ti deposition onto a series of several wafers.
Specific process parameters that are particularly preferred are set forth in detail in the examples and specific embodiments described below.
With the stabilization of material deposited on reactor components in the chamber carried out in a controlled manner, the reactor surfaces reach a conditioned state in the reactor in a very short time, much shorter than would be required otherwise, and changes in process conditions during the performance of Ti-PECVD processes are avoided. While stabilization may occur slowly if left to itself due to reaction with residual water vapor in the chamber, the controlled use of oxidizing or reducing and passivating agents during the process stabilizes the film quickly and in a repeatable manner. In particular, the time required to form a stabilized TiClx based coating on reactor surfaces is controlled and significantly reduced. Further, the stabilization of the accumulating film after each deposition during the continuous operation of the reactor renders the process stable for the processing of thousands of wafers. This method is preferred over methods that rely on frequent in situ cleaning and recovery from in situ cleaning.
In particular, from the onset of deposition, such as, for example Ti-CVD, after either a wet clean or chemical cleaning, such as by using NF3, material is deposited on the chamber walls and oxidized or reduced/nitridized into a fully insulating and chemically stable film while avoiding unstable operation or, alternatively, the need for a lengthy break-in period. Typically, following reactor cleaning, stabilization occurs over a period of time comparable to the time it takes for the CVD processing of less than five wafers, typically 1 wafer, where previously about 75 conditioning wafers were required before resuming commercial manufacturing processing of wafers. Even after a long idle time, for example after an overnight shutdown of the equipment, break-in time is typically reduced to zero wafers.
Also, without the stabilizing step of the present invention being performed following the processing of individual wafers, instabilities persist throughout the continuous operation of the reactor. Such passivation of coatings deposited on reactor components between depositions onto wafers in the commercial process enhances process and reactor stabilization. In the case of Ti-CVD, where each deposition onto a wafer results in the deposition of additional metallic titanium onto reactor components, formation of a passivating film on the titanium on the reactor components can be carried out by a treatment of approximately 10 to 60 seconds, typically 30 seconds, with an NH3 plasma between each consecutive process of Ti-CVD onto wafers. This maintains consistent film properties, such as resistivity and film thickness uniformity, as well as consistent deposition rate.
The concepts set forth above have utility for other metal inorganic CVD processes, particularly those used in applying reactive boundary materials in addition to titanium, such as tantalum.